Modern process control targets are subject to process developments designed for smaller features. For instance, optical metrology targets are being fabricated with thinner layers and materials with different optical constants. Developments in target design and other factors increase measurement sensitivity to the spectral content of probing beams. Accordingly, spectrally controlled illumination is often employed to enable monitoring and control of the effect of wavelength on measurement.
In some imaging and angular scatterometry systems, spectral bands are selected using filters. However, a bandwidth of a few tens of nanometers is required in order to maintain adequate precision*MAM performance. Spectroscopic scatterometers often include spectrometers for detection of illumination spectra. Imaging and angular scatterometry suffer from a limited choice of spectral structures which limits measurement potential because of the limited number of wavelength structures available per target. Similar limitations apply for spectroscopic scatterometry, because of the limited number of angular structures available for selection. There is a need in the art for increased configurability in spectral control systems.